The present invention relates to a low refractive index SiO2 film which is applicable to a light antireflection film on various surfaces of curve mirrors, back mirrors, goggles, window glasses, displays of personal computers and word processors, and other various commercial displays.
In recent years, antireflection films have been applied to surfaces of various displays of word processors, computers, and televisions, surfaces of various optical lenses and optical articles, and surfaces of window glasses of automobiles and electric railcars to prevent the reflection of light from the above surfaces.
An about 0.1 μm-thick thin film of MgF2 formed on glass will be described as an example of the antireflection film. It is already known that, when incident light perpendicularly enters a thin film, in order for the antireflection film to prevent the reflection of light by 100% and to pass light by 100% therethrough, relationships represented by the equations (1) and (2) should be met (see “Science Library” Physics=9 “Optics,” pp.70-72, 1980, Science Sha Ltd., Japan).n0=√{square root over (ng)}  (1)n0h=λ0/4  (2)wherein λ0 represents a specific wavelength, no represents the refractive index of the antireflection film at this wavelength, h represents the thickness of the antireflection film, and ng represents the refractive index of the substrate.
It is already known that the refractive index ng of glass is about 1.5, the refractive index n0 of MgF2 film is 1.38, and the wavelength λ0 of incident light is 5500 Å (reference). When these values are substituted in the equation (2), the results of calculation show that the thickness h of the antireflection film is about 0.1 μm which is the thickness of an optical thin film. For this reason, an optical thin film having such thickness has been used as the antireflection film. Vacuum process, such as vacuum deposition, sputtering, ion plating, and plasma CVD, have been known to be suitable for the formation of such an optical thin film.
From the equation (1), it is apparent that prevention of the reflection of light by 100% can be attained by the selection of such a material that the refractive index of the upper coating is approximately equal to a value of square root of the refractive index of the lower coating. The antireflection of light by utilizing the above principle to make the refractive index of the upper coating lower than the refractive index of the lower coating, that is, by providing a thin film of a high refractive index layer and a thin film of a low refractive index layer in that order on a substrate, has hitherto been performed in the art.
An SiO2 film is generally known as a low refractive index film and hence has been extensively used as antireflection films and the like. Antireflection films having various layer constructions, however, have been desired to have lower refractive index.